Fuel oil or various liquid chemicals are stored in a tank. In recent years, for example, a centralized oiling system for collective housing has been proposed. In this system, kerosene is supplied to the respective homes from a centralized kerosene tank through tubes.
The tank may suffer some cracks due to time degradation. In this case, liquid in the tank leaks from the tank. It is very important to detect such leakage as soon as possible and cope with it adequately for preventing explosion and fire hazard, ambient pollution, or generation of poisonous gas.
As a device for detecting leakage of liquid in a tank in the shortest possible time, JP-A-2003-185522 (Patent Document 1) has proposed a configuration that includes a measuring pipe or measuring tube into which liquid in a tank is introduced and a measuring slim-tube provided below the measuring tube and measures the flow rate of liquid inside the measuring slim-tube using a sensor section additionally provided to the measuring slim-tube to detect a minute variation of the liquid surface in the tank, i.e., a liquid level variation.
In this liquid leakage detection device, an indirectly heated flowmeter is used as a sensor additionally provided to the measuring slim-tube. In this flowmeter, an electric current is applied to a heating element to generate heat, and a part of the heating value is allowed to be absorbed by the liquid. Then, the heat absorption value of the liquid varies in accordance with the liquid flow rate. This characteristic is used to detect influence of the heat absorption based on a variation in an electrical characteristic value such as a resistance value represented by a temperature variation of a temperature-sensitive element.
However, in the indirectly heated flowmeter used in the liquid leakage detection device disclosed in the above Patent Document 1, a variation in an electric circuit output level with respect to a variation in a liquid flow rate becomes small in the region where the flow rate value is as infinitesimal as, e.g., 1 milliliter/h or less, so that an error in the flow rate measurement value tends to increase. Thus, there is a limit to an improvement in leakage detection accuracy.
Assuming that an external power source is used as a power source for current application to a heating element in the above configuration, a power wire needs to be laid from outside to a sensor in the liquid leakage detection device. There is a possibility that an electrical leakage may occur in such a power wire particularly at the lead-in portion to the structural section of the liquid leakage detection device over a long-term use. In the case where a flammable liquid or an electrically conductive liquid is used, a fire or short-circuit associated with the electrical leakage may be caused on the liquid adhered to the structural section of the liquid leakage detection device in some cases.
From the above viewpoint, it is preferable to use a battery installed in the structural section of the liquid leakage detection device as a power source for a heating element of the sensor, particularly in the case where a flammable liquid or an electrically conductive liquid is used. In this case, it is desirable to reduce power consumption of the liquid leakage detection device in order to perform leakage detection without requiring battery replacement as long as possible.
It is also possible to detect leakage of liquid in a tank based on the liquid level variation. The liquid level variation can be measured by using a pressure sensor. Since the pressure sensor detects the liquid level variation by converting a detected liquid pressure to a depth from liquid surface to the pressure sensor, specific gravity of liquid to be measured is involved in the conversion. Therefore, in the case where leakage detection is performed for only liquid to be measured having a constant specific gravity (e.g., water), it is possible to promptly obtain a liquid level value based on an output from the pressure sensor by previously inputting the specific gravity value of the liquid to a conversion program and thereby leakage detection can accurately be performed based on the obtained liquid level value.
However, in the case where the liquid to be measured is a mixed composition containing many organic compounds such as fuel oil (gasoline, naphtha, kerosene, diesel oil, or heavy oil), even if the composition contains the same kind of fuel oil (e.g., kerosene), various kerosenes having different specific gravities (e.g., a specific gravity difference of 0.05) exist in the composition because the compound composition of kerosene differs depending on distillation condition at purification process.
Therefore, when leakage detection is performed for, e.g., kerosene contained in a tank, even if a specific gravity value of standard kerosene is input, as a specific gravity of liquid, to the conversion program from a liquid pressure to liquid level, an error occurs in the conversion when the kerosene actually contained in a tank has a different specific gravity value from the standard kerosene. When the amount of the kerosene in a tank becomes small, the tank is replenished with fresh kerosene. At this time, various types of kerosenes can be added to the tank and, accordingly, the specific gravity value of the kerosene contained in the tank may differ with each replenishment in some cases. Thus, in the case where leakage detection is performed for the mixed composition such as kerosene, liquid level measurement includes the above conversion error, resulting in a reduction of accuracy in leakage detection.
Patent Document 1: JP-A-2003-185522